A high temperature superconductor based fault current limiter (hts-fcl) is a device that automatically limits fault currents in high voltage networks to a low current value close to the nominal current. The benefit of such a device is that it reduces drastically the short circuit power of the high voltage network and, thus, allows to interconnect networks without increasing the short circuit power or to decrease safety margines so that other machineries connected to the network can be designed for lower short circuit power and, therefore, can be made lighter and cheaper.
A hts-fcl makes use of the fact that the high temperature superconductor material looses its superconductivity and transits from the non-resistive superconducting state to a normal state with high electrical resistivity when at least one of the critical current (Ic), the critical temperature (Tc) or the critical magnetic field (Hc) of the superconductor material is exceeded. This transition from the superconducting state to the normal resistive state is called “quenching”.
In normal operation with nominal current In, that is, in the cooled state, the superconductor material is in its superconducting state with essentially zero resistance so that there is no voltage over the whole fcl—the fcl is “invisible” for the network.
In case of fault such as short circuit the current rises to several times the nominal current In exceeding Ic of the superconductor material which causes the superconducting material to transit to the normal resistive state.
The electric resistance R of a fcl in the normal resistive state is generally chosen such that the current which can pass the hts components is not more than about 2 to 6 times, preferably 2 to 3 times, of the nominal current In.
For example, in a 110 kV network the fcl should limit the current to 5.400 ampere in fault event which corresponds to the threefold of the nominal current of 1.800 ampere. Consequently the hts components should have a normal state resistance of about 20 ohms with Rfcl=Voltage/limited current.
There are known hts-fcl which are composed of a bulk component of high temperature superconductor material or of a plurality of such bulk components electrically connected in series.
Within a resistive fcl with the hts components electrically connected in series the components must be very homogenous in view of their properties such as critical current density, critical current, normal state resistivity etc. If, for example, the critical current Ic of the hts-components or of part of a single component differs quench and in the result heat up is non uniform. Such non uniform heat up would lead to the formation of a temperature gradient within the material of the hts components and in the result to breakage due to thermal shock.
Further, if only the part with lowest Ic starts to quench resistance built up in this part is insufficient to limit the fault current until also the other hts components reach their resistive state. In the consequence without the provision of suitable means such as a breaker this part will heat up and local burn-out can occur.
In general, during fault event when the high temperature superconductor material quenches the material has to absorb a large quantity of energy within a very short time of only some tens of millisecond. In the result within the components a power density is generated which is orders of magnitude too high to be transported into a cooling environment, usually cooling bath, within the necessary short time. Since sufficient heat dissipation is not possible the hts material heats up almost adiabatically until melting of the material. Thus, for avoiding heating up until melting of the material a breaker or likewise means must be provided for taking the fcl from the network.
U.S. Pat. No. 5,761,017 discloses a fault current limiter comprising high temperature superconductor filaments encapsulated in an epoxy having thermal conductivity properties that enable the superconductor do heat rapidly during fault while preventing adiabatic heat up until damage. The epoxy encapsulation serves to dissipate heat from the filaments into the encapsulation in order to avoid adiabatic heat up. On the other hand the epoxy encapsulation must have large heat capacity and insulates the superconductor filaments.
By this thermal insulation effect of the epoxy encapsulation the temperature to which the superconductor filaments are subjected is kept considerably above the critical temperature for decreasing the critical current density. Due to the decrease of critical current density quenching is promoted and overall resistance obtainable enhanced. In the consequence the epoxy encapsulation serves to thermally insulate the filaments from the cooling bath.
A further drawback of known fault current limiters is, that due to the very high power density generated inside the hts material the material can not recover to the superconducting state “under load”, that is online, after quench but must be taken from the network for cooling down. However, in the result, when the hts-fcl is offline in order to avoid melting and/or for recovery to the superconducting state the network is not protected against fault events or any other additional protection equipment would be necessary.
Apart from the use as bulk material referred to above there is also known to use hts-material in form of layers, wherein a layer of the hts-material is deposited on a suitable substrate.
However, for a superconductor layer to have good superconductor properties such as high current density etc. the crystal grains of the superconductor material must have a certain degree of orientation. In particular, the crystal grains should be aligned both perpendicular to the plane of the substrate (c-axis orientation) and parallel to the plane of the substrate (a-b-orientation).
In general, techniques for obtaining hts-layers with suitable alignment (texture) are well known.
For example, there are vacuum based processes such as pulsed laser deposition, sputtering and electron beam evaporation.
A specific example for such vacuum based process is a method called IBAD (ion beam assisted deposition) wherein on randomly oriented metallic substrates a highly textured buffer layer is deposited which serves to transfer the desired texture to the superconductor layer grown on the buffer layer.
Further, according to ISD—inclined substrate deposition—texturing of a to be deposited layer is obtained by deposition under specific angle.
In addition, there are wet chemical processes such as metal organic deposition (MOD) including sol-gel method etc.
In MOD-techniques usually organic compounds of the metals constituting the superconductor material are used as precursors, dissolved in a solvent, deposited on the substrate and converted to the final hts-material by heat treatment.
Substrates with suitable texture are obtainable by RABITs (rolling assisted biaxial texturing of substrates). Such textured substrates can serve as template for transferring a desired texture to a layer deposited thereon.